WO2010089161A1

WO2010089161A1 - Brake system for motor vehicles

Info

Publication number: WO2010089161A1
Application number: PCT/EP2010/050085
Authority: WO
Inventors: Stefan A. Drumm
Original assignee: Continental Teves Ag & Co. Ohg
Priority date: 2009-02-05
Filing date: 2010-01-07
Publication date: 2010-08-12
Also published as: DE102009024035A1; KR101591129B1; DE102009024034A1; EP2393696B1; EP2393696A1; KR20110116040A; US20110291469A1; WO2010089162A1; CN102307762A; US8702181B2; CN102307762B

Abstract

The invention relates to a brake system for motor vehicles that can be actuated in a "brake-by-wire" operating mode both by the vehicle driver and independently of the vehicle driver, and that is operated preferably in the "brake-by-wire" operating mode, and that can be operated in a fallback operating mode wherein only operation by the vehicle driver is possible. The brake system comprises a first piston (2) connected to a brake pedal (1) and guided in a second or a retaining piston (3) bounding a retaining chamber (4) in a housing (20). Separating piston devices (5a, b) interacting with the retaining piston (3) are also provided and are connected to the vehicle wheel brakes (8 - 11). In order to achieve a construction of the brake system that can be implemented simply, the invention proposes that the retaining chamber (4) bounded by the second piston (3) is designed as an annular chamber radially encompassing the actuating axis defined by the axes of symmetry of the first and second pistons (2, 3).

Description

Brake system for motor vehicles

The present invention relates to a brake system for motor vehicles, which is drivable in a "brake-by-wire" mode so well ^¬ by the driver and independent of the driver, preferably in the "brake-by-wire" Be ^¬ mode of operation and in at least one Rückfallbe ^¬ can be operated mode in which only the operation by the driver is possible, with

a) a brake pedal for actuating a first hydraulic devices ^¬ rule piston) is applied to the when actuating the braking system (by the vehicle operator an operating force of the pedal force and in a in a

Housing a hydraulic retention space limiting, second hydraulic (retaining) piston is slidably guided,

b) a position detecting device that detects the actuating travel of the first piston,

c) a pressure medium reservoir,

d) a hydraulic circuit for brake booster, essentially with an electrically controllable, hydraulic pressure source, a loadable from the pressure source high-pressure accumulator, the pressure of the wheel brakes in the "brake-by-wire" mode act ^¬ bar, and with a pressure control valve assembly, e) solenoid valves for electronically controlled modification of brake system pressure,

f) between the high-pressure accumulator and the wheel brakes connected, electrically controllable pressure control valves,

g) as well as separating piston devices, which in the "brake-by-wire" operating mode are acted upon by the pressure set by the pressure regulating valves on a first separating piston surface and transmit the pressure exerted by a second separating piston surface to the wheel brakes,

h) can be brought with the second piston in a force-transmitting connection actuation elements for the mechanical control of the separating piston devices and

i) an electronic control unit.

Such a brake system is z. B. from international patent application WO 01/72567 Al known. A disadvantage is felt in the known brake system, the type of mechanical control of the separating piston devices by the second piston, which takes place by means of a balance beam, which is laterally coupled to the separating piston. In a power transmission from the second piston this comes to rest on the balance beam, so that the separating piston devices are moved in the direction of actuation in the sense of a pressure build-up in the wheel brakes. The lateral coupling of the balance beam to the separating piston is considered to be an avoidable complication of the structural design. In addition, the energy efficiency of the prior art brake system in two points in need of improvement. In the preferred brake-by-wire operating mode, even lower decelerations, which do not require high dynamics and no wheel-specific adaptation of the wheel brake pressures, are realized with a pressure fluid volume taken from the high-pressure accumulator. Thereafter, the storage must be recharged with a significantly higher energy expenditure compared to the hydraulic energy used. This applies to the majority of the braking occurring during average driving. Therefore, from the point of view of energy consumption, it makes sense to use pressure medium from the hydraulic accumulator only if dynamic braking is required and preferably to carry out braking directly with the aid of the pressure of the electrically controllable pressure source.

In a fallback mode in which no electrical control, but hydraulic energy in the form of pressure and pressure medium volume in the hydraulic accumulator is available, the known brake system works (as well as in the brake-by-wire "mode) as a power brake. It is therefore more appropriate that the new brake system proposed here operates as an auxiliary power brake in this operating situation, for which purpose the restraint must be switched off, which is not the case in the previously known system is possible.

For both points to improve energy efficiency, the present invention provides the technical requirements.

It is therefore an object of the present invention to provide the design conditions for an increase in energy efficiency and at the same time a simplification - A -

to achieve the construction of the brake system of the type mentioned or to propose a brake system, in particular in the field of power transmission between the second piston and the separating piston devices a significant design simplification and improved functionality is achieved.

This object is achieved in that limited by the second piston retaining space is formed as an annular space which engages around the defined by the symmetry axes of the first and second piston actuating axis radially.

Preferably, the end face of the second piston pointing in the direction of actuation plunges into an unpressurized space formed in the housing, wherein the non-pressurized space particularly preferably accommodates sections of the actuation elements which enable the mechanical transmission of force from the second piston to the isolating piston devices.

Preferably, a return spring for biasing the second piston is arranged opposite to the actuating direction in the non-pressurized space.

Advantageously, the control elements are designed as round, sealed in the housing and guided parallel to the actuating axis rods.

Preferably, the control elements can be brought into a force-transmitting connection with the second piston. Furthermore, damping means are provided between the second piston and the actuation elements. The control elements are preferably acted counter to the direction of actuation with the wheel brake, by their action they are held in contact with the second piston.

According to one embodiment of the invention, the first piston in the second piston limits a hydraulic Nachlaufräum, which is designed as the first piston radially encompassing annular space.

In this case, the follow-up space is preferably connected to the pressure medium reservoir.

Preferably, the follow-up space is connected to the non-pressurized space and thus indirectly to the pressure medium reservoir.

According to one embodiment of the invention, two seals are adjacent to each other at the end facing the brake pedal of the second piston defining an annular chamber, wherein the first seal seals the annular chamber relative to the retention space and the second seal seals the annular chamber from the atmosphere.

Advantageously, the annular chamber is connected to the follow-up space.

According to a development of the invention, the first piston in the second piston limits a control room, in which it can build up a hydraulic control pressure that can be controlled via a brake pedal force. Preferably, the drive pressure is passed through a connection channel penetrating the second piston in the radial direction into the housing.

It is likewise preferred that the triggering pressure is detected by a pressure sensor arranged fixed to the housing as a signal representing the brake pedal actuating force.

The control pressure is advantageously a hydraulic control of the pressure control valve assembly.

According to one embodiment of the invention, the pressure control valve assembly is designed as a hydraulically controlled 3/3-way valve.

Preferably, a valve arrangement is provided, which allows aufzuschalten either the control pressure or the pressure of the pressure medium reservoir to the hydraulic control port of the pressure control valve assembly.

Likewise, it is preferred that a further valve arrangement is provided, which allows to separate the hydraulic connection of the pressure regulating valve arrangement to the pressure medium reservoir and instead turn on the output from the controllable pressure source pressure.

According to one embodiment of the invention, the path detection device is designed as a contactless path detection device which cooperates with a coupled to the first piston position sensor.

Particularly preferably, the position sensor is formed by a permanent magnet which is connected to the first piston by means of a cantilever, which extends through one in the second Piston formed pressure-free channel extending therethrough, which is parallel to the actuating axis.

According to one embodiment of the brake system according to the invention, a hydraulically actuated travel simulator is provided, which gives the vehicle driver the usual pedal feel in the "brake-by-wire" mode.

The travel simulator is preferably connected via a hydraulic line to the control room, which can be shut off by means of a preferably electromagnetically actuated isolation valve.

It is likewise preferred that the travel simulator is arranged outside the second piston.

According to a preferred embodiment of the brake system, the pressure source is formed by an electro-hydraulic actuator.

Advantageously, the electrohydraulic actuator has an actuator piston driven by an electric motor, wherein the travel path of the electrohydraulic actuator piston is particularly preferably detected by means of a displacement sensor.

According to another preferred embodiment of the brake system, the pressure source is formed by an electric motor driven pump.

Further advantageous developments of the subject invention are listed in the dependent claims.

In the following description, the present invention will be described explained with reference to the accompanying schematic drawing of an embodiment. In the drawing, the single figure shows the structure of an embodiment of the brake system according to the invention.

The brake system shown only schematically in the figure essentially has a first hydraulic piston 2 which can be actuated by means of a brake pedal 1 and which is displaceably guided in a second hydraulic piston 3 of larger diameter, which is also referred to as a retaining piston, and its annular surface 82 in a housing 20 a hydraulic retention space 4 bounded, which surrounds the retaining piston 3 radially. The brake system according to the invention further comprises the retaining piston 3 operatively downstream separating piston devices 5a-d, which electrically upstream pressure control valves 6a-d, 7a-d are connected and connected to the vehicle brakes 8-11, a pressure fluid reservoir 12, a travel simulator 13, a hydraulic pressure source 14, a high-pressure accumulator 15, and an electronic control and regulation unit 16. In this case, each wheel brake 8-11 is preferably assigned a separating piston device 5a-5d. A force transmission between the retaining piston 3 and the separating piston devices 5a, 5b takes place by means of mechanical control elements 17a-d which are designed as force transmission rods and which are supported on the retaining piston 3 with the interposition of damping elements 22a-d. The mechanical control elements 17a-d preferably extend through an unpressurized space 19, which is connected to the pressure medium reservoir 12 and receives a return spring 21 which biases the retaining piston 3 against the actuation direction. To seal the control elements 17a-d in the housing 20 seals are provided, which are provided with the reference numeral 18a-d. As the single FIGURE further shows, the first piston 2 in the second or restraining piston 3 delimits a hydraulic control space 23 which, on the one hand, interposes an electromagnetically operable 2/2-way valve or check valve 24 with the travel simulator 13 and, on the other hand, via a currentless open (SO) 2/2-way valve 25 is connected to a hydraulic control port of a pressure control valve assembly 26, which will be explained in more detail in the following text. The aforementioned drive connection can be connected via the pressureless space 19 to the pressure medium reservoir 12 with the interposition of a normally closed (SG) 2/2-way valve 27. The hydraulic drive chamber 23 receives a second return spring 28 which serves to bias the first piston 2 against the actuation direction and the return of the brake pedal 1 coupled to the first piston 2.

The pressure controlled in the control room 23 pressure is detected by a pressure sensor 29, and the actuation path of the first piston 2 is detected by means of a non-contact position sensing device 30 which cooperates with a coupled to the first piston 2 permanent magnet 31. For coupling the permanent magnet 31 to the first piston 2, a boom 32 is provided which extends in a formed in the second piston 4 pressure medium passage 33 which connects a limited in the second piston 3 by the first piston 2 Nachlaufräum 34 with the unpressurized space 19. In addition, at the brake pedal 1 facing the end of the second piston 3, two seals 78, 79 arranged side by side, which limit an annular chamber 80. The first seal 78 seals the annular chamber 80 from the retention space 4, while the second seal 79 seals the annular chamber 80 from the atmosphere. A formed in the retaining piston 3 pressure fluid channel 81 connects the annular chamber 80th This ensures that the pressure-bearing seal 78 is wetted from both sides with pressure medium and the sealing gasket 79 is exposed to the environment no differential pressure.

Each wheel brake is associated with a pressure control valve assembly - either as shown, constructed from two 2/2-way valves or alternatively as a 3/3-way valve. It is crucial that each wheel-specific pressure control assembly has three hydraulic connections. A normally closed "high pressure port", an outlet port and a "low pressure port". The high pressure connections are connected together to the high pressure accumulator. The output connections are individually to the separating piston devices 5a-d and the low-pressure connections are connected in common to the line section 46, which can be acted upon by a brake system pressure and in the illustrated rest state of the brake system via the pressure control valve assembly 26, the normally open valve 43 and the unpressurized space 19 to the also unpressurized pressure fluid reservoir 12 is connected.

The aforementioned normally open (SO) 2/2-way valve 43, together with a normally closed (SG) 2/2-way valve 42, a valve assembly with three hydraulic connections: a connected to the unpressurized space 19 or the pressure medium reservoir 12 Niederdruckan- conclusion, connected to a low pressure port of the pressure regulating valve assembly 26 working port and a high pressure port which is connected to a hydraulic line section 41, via which - as will be explained in more detail - a hydraulic pressure can be fed. As an alternative to the SO-SG 2/2 valve pair 43, 42 formed valve assembly with three terminals can single 3/3-way valve can be used.

Furthermore, it can be seen in the drawing that the previously mentioned pressure source 14 is formed as an electro-hydraulic actuator whose piston 36 can be actuated by means of a schematically indicated electric motor 35 via a rotational-translation gear. In this case, the distance traveled by the piston 36 path is monitored by a displacement sensor bearing the reference numeral 37. The limited by the piston 36 pressure chamber 38 is connected via a suction check valve 44, the hydraulic connecting line 45 and the unpressurized space 19 with the pressure fluid container 12 and a discharge check valve 40 to the high-pressure accumulator 15 and further via a usable in both directions of flow hydraulic connection line 41 with a solenoid valve 42. As an alternative to the use of an electro-hydraulic actuator is in another, not shown embodiment of the invention, the pressure source 14 designed as an electric motor driven pump, which is inserted instead of the check valve 44 in the hydraulic circuit. The suction side of the pump is connected via the hydraulic connecting line 45 and the unpressurized space 19 with the pressure medium container 12 and the discharge side of the pump with the check valve 40 and the hydraulic connecting line 41. The pressure source 14 remote from the discharge check valve 40 is connected via a pressure medium line 39 to input terminals of the first pressure control valve group 6a - 6d, to which the aforementioned high pressure accumulator 15 is connected so that it is charged via the discharge check valve 40 from the pressure source 14 can be. The detection of the pressure held in the high pressure accumulator 15 pressure is a pressure sensor 49. The aforementioned check valve 40 prevents emptying of the high pressure accumulator upon a withdrawal of the pressure This allows adjusting a pressure value below the pressure level of the high-pressure accumulator 15 in the hydraulic connection line 41 by means of the electronically controllable pressure source 14. About a current of said valve assembly (42, 43) or equivalent to this assembly 3/3 way valve can the pressure source 14 via the pressure control valve assembly 26 are connected to the line section 46 so that the pressure source 14 can be directly connected as system pressure to the wheel brakes without the inevitable in the prior art "detour" over the high-pressure accumulator and the energy consuming overcoming unnecessary pressure differences. .

In the comparatively rare braking situations in which a particularly high dynamics of the brake pressure build-up and / or different brake pressures are required, in addition to the already described, job-saving direct construction of a system brake pressure via the pressure control valves 6a-d under high pressure pressure medium can be retrieved, which corresponds to an indirect pressure build-up, because previously the high-pressure accumulator had to be charged by means of the pressure source 14.

For sucking in pressure medium by the controllable electric pressure source 14 from the pressure fluid reservoir 12 is a direct hydraulic connection line 45 from the pressure fluid reservoir 12 and the unpressurized space 19 connected thereto

A check valve 47 provides a Nachsaugverbindung of pressure fluid reservoir 12 to the braking system pressure in the line section 46, which allows a Nachsaugen of pressure fluid in a mechanical actuation of the separating piston 55 a, 55 b. In addition, the already mentioned, for pressure fluid reservoir 12 closing check valve 47, the output port of the aforementioned pressure control valve assembly 26, a pressure sensor 48 with a brake system pressure representing the output signal and a second hydraulic control port of the pressure regulating valve assembly 26 are connected to the line section 46.

As one skilled in the art can readily deduce, the pressure regulating valve arrangement 26 mentioned above is designed as a 3/3-way valve which acts in the sense of a pressure compensator with oppositely directed directions of action, on the one hand, from the pressure actuated by the driver in the control room 23 and, on the other hand, by the control unit Line section 46 prevailing brake system pressure via two in the drawing right and left arranged hydraulic control connections can be controlled. To realize the pressure regulating valve arrangement 26 illustrated in the drawing as a 3/3-way valve, both the slide valve technology and the seat valve technique or a combination of these variants can be used. It should be expressly pointed out that between the figuratively illustrated switching positions of the pressure regulating valve assembly 26 with changing hydraulic passages and closures continuous transition states exist, which can be stably approached with appropriate control in terms of control technology. The illustrated 3/3-way valve is therefore designed as an analog valve and not as a switching valve with unstable transition states. If in the text below nevertheless switching positions is mentioned, then it is meant each a Ansteuerungs- or actuation state of the pressure control valve assembly 26, although continuously variable one of the valve positions shown figuratively comes close or can be assigned. In addition to the already In the first, shown in the drawing switching position is its working port (A) via its low pressure port (T) with the center tap of the two 2/2 -Wegeventile 42, 43, while its high pressure port (P), which is connected via a further line section 50 to the pressure medium line 39 and thus to the input terminals of the first pressure control valve group 6a - 6d remains locked. In the second switching position of the 3/3-way valve 26, all three connections (P, T, A) remain blocked while in the third switching position the working connection (A) is connected to the high-pressure connection (P). The aforementioned components 14, 15, 35-38, 26, 39-50 form a hydraulic circuit for the electronically controlled provision of individual wheel brake pressures, in particular in response to a brake force introduced into the system via the brake pedal 1 in the brake-by-wire mode. In addition, when hydraulic control is not available via the abovementioned pressure regulating valve arrangement 26, reinforced brake pressures are provided, whereas with exhausted high-pressure accumulator 15, only unamplified wheel brake pressures are available.

The above-mentioned travel simulator 13, which gives the driver in brake-by-wire mode the usual brake pedal feel, consists of a preferred, but not necessarily binding for the invention embodiment of a simulator piston 53 and two elastic elements 51, 52, the be deformed by a hydraulic displacement of the simulator piston 53.

For a better overview, only the first two separating col- beneinrichtungen 5a and 5b shown and described in detail. The separating piston devices 5a, 5b each comprise a separating piston 55a, b, which in each case has a first separating piston surface 65a, b and a second separating piston surface 75a, b, and which are biased counter to their actuating direction by a respective compression spring 76a, b. The pressure-related allocation of the separating piston surfaces 65a, b and 75a, b is preferably such that the first separating piston surface 65a, b can be acted upon by the pressure (wheel-individual pressure) set by the pressure regulating valves 6d, 6c, 7d, 7c, while the separating piston surface 75a , b the construction of a hydraulic pressure in the wheel brakes 8 - 11 is used. From the separating piston 55 a, b limited pressure chambers 77 a, b are connected by means of shut-off hydraulic connections with pressure fluid reservoirs, which may preferably form a unit, not shown with the pressure fluid reservoir 12, whereby a limited exchange of pressure medium between the otherwise separate hydraulic circuits is made possible.

The brake system has, for example, an electrical memory 90 and a voltage converter 91.

In the following text, the operation of the brake system according to the invention will be explained in more detail on the embodiment shown in FIG.

In the unactuated state of the brake pedal 1, the simulator piston 53 is pressed by the elastic elements 51, 52 to a closer not designated stop, the first piston 2 together with the brake pedal 1 coupled thereto held by a prestressed spring 28 in a rest position, the second piston (retaining piston ) 3 of a prestressed spring 21 at a stop in the housing 20 and the separating piston beneinrichtungen of the said compression springs 76 a, b pressed on the second piston 3. In the preferred brake-by-wire mode, movement of the brake pedal 1 is hydraulically coupled to movement of the simulator piston 53 by hydraulically preventing the second piston 3 from moving by closing the valve 54. The pressure required for deformation of the elastic elements 51, 52 in the travel simulator 13 is detected with the aid of a pressure sensor 29 as well as the path of the first piston 2 coupled to the brake pedal 1 by means of the position detection device 30, 31. From these two detected signals, the represent the brake pedal force and the brake pedal travel, the driver deceleration request is determined in the only schematically illustrated e- electronic control unit 16. From this delay request, a system pressure value is calculated. In a basic function, this calculation consists of a simple multiplication of the pressure value detected by the pressure sensor 29 by a factor which is such that the sum of the pressure forces acting on the separating piston surfaces 65a, b multiplies the force exerted by the brake pedal 1 on the first piston 2 with the gain V (common values are V approximately equal to 4, usually in the range of 3 to 7) of a conventional brake booster. While in the conventional brake booster this connection is determined constructively and thus can not be adapted to the operating situation, a system pressure value deviating from the basic function can be generated here by a possible change of the calculation method at any time. For example, both the so-called Springerfunktion (increase in the system pressure value at low pedal travel) and the brake assist function (increase of the system pressure value at high pedal speed) can be easily realized in the form of a numerical algorithm, while these functions in the It is also possible to specify system pressure values for driver assistance that deviate from the driver's specification, for example for automatic braking to the automatic distance or for collision avoidance, which corresponds to exceeding the driver's specification Another example is recuperative braking, in which a driver's braking request is wholly or partially realized with the aid of the vehicle drive, so that the system pressure value can be lowered or set to zero.

Finally, if required, individual wheel brake pressure values are derived from the system pressure value initially provided jointly for all wheel brakes-for example with the aid of ABS, ASR and ESP control algorithms for avoiding blocking and spinning wheels and vehicle spin or vehicle overturning.

To pressurize the wheel brakes with the system pressure value, this pressure can be supplied via the line section 41, the energized valve 42, the unactuated pressure regulating valve arrangement 26, the line section 46, the valves 7a to 7d and the separating piston devices 5a, b in all wheel brakes via electronic control of the pressure source 14 become. If required wheel-individual pressures, however, the hydraulic path from the high-pressure accumulator 15 via the line section 39 to the valves 6a to 6d and from there again via the separating piston devices to the wheel brakes is provided. If the pressure of a wheel brake is to be reduced, this is possible by opening the corresponding valve 7a, 7b, 7c or 7d, the line section 46, the unactuated pressure regulating valve arrangement 26, the de-energized valve 43 and the unpressurized space 19. In the mentioned Pressure control operations in brake-by-wire mode, the restraining piston 3 is held by the system pressure to a stop, not shown in the housing 20, so that the driver upon actuation of the brake pedal 1 a pedal feeling is mediated by the elastic elements 51, 52 of the travel simulator 13th is predetermined.

During a power failure, which is caused for example by a battery failure, a short circuit or by switching off the ignition, the brake system according to the invention automatically goes into a first fallback mode in which hydraulically amplified driver braking is possible. When actuated by the driver, a pressure is built up in the control room 23, which acts on the non-energized valve 25 as a hydraulic control pressure for the pressure control valve assembly 26. This increases its output pressure in the line section 46 until it has reached a value proportional to the control, whereby the proportionality factor within the pressure regulating valve arrangement 26 is determined constructively. In this case, the output pressure value acts as a system pressure value with which all wheel brakes are subjected equally. In this fallback mode, the retaining piston 3 initially moves away from the stop. However, he does not get far, because the system pressure is increased until the retaining piston 3 is pushed back into the vicinity of the stop. When the driver control is released, the system pressure is correspondingly reduced via the pressure regulating valve arrangement 26. The pedal characteristic is practically the same as in the brake-by-wire mode, with only the small displacement of the restraining piston 3 from the position at the stop in the position near the stop cause small, barely perceptible by the driver changes the pedal characteristics. Third-party braking required for ASR, ESP and ACC are not possible without electronic intervention. The transmission behavior of the brake system is characterized by a linear brake boost without Springerfunktion. When a power failure occurs during braking, there are no irritating pedal force changes and movements.

In case of failure of the external pressure - for example, when exhausted high-pressure accumulator 15, the brake system according to the invention automatically goes into a second fallback mode. In this mode, the retaining piston 3 moves in the drawing to the left. In this case, the previously described mechanically actuable control elements 17a, 17b, which transmit the movement of the restraining piston 3 to the separating pistons 55a, b. The transmission preferably takes place in such a way that the separating piston paths are equal to the path of the retaining piston 3. By appropriate design dimensioning, when the second fallback level is utilized, the pedal characteristic changes to an extent that signals the driver that the brake system is in an emergency mode, with the change in pedal characteristic being designed so that the driver is not greatly disturbed ,

The proposed brake system is safer by the majority of automatically responsive fallback modes and four hydrostatic Radbremskreise than all today built in series braking systems, and with the same, if not lower cost of space, weight and number of technically demanding parts such as valves, pistons and seals. As a "brake-by-wire" brake system, it fulfills the current requirements for integrated brake force amplification with freely definable characteristic curve, for example with "jumper function", external controllability and pedal backdriving. freedom from electronic interference with brake pressure control.

Claims

claims

1. Braking system for motor vehicles, which is controllable in a "brake-by-wire" mode both by the driver and independently of the driver, preferably in the "brake-by-wire" mode is operated and can be operated in at least one fallback mode, in which only the operation by the driver is possible, with a) a brake pedal (1) for actuating a first hydraulic piston (2) on the actuation of the brake system by the driver an actuating force (pedal force) is exercised and in one in a housing (20), a hydraulic retaining space (4) limiting, second hydraulic (retaining) piston (3) along a Be ^¬ tätigungsachse is slidably guided, b) a displacement detection device (30, 31), the actuating travel of the first piston ( 2), c) a pressure fluid supply reservoir (12), a pressure supply device, essentially consisting of an electrical control battery Ren, hydraulic pressure source (14) and one of the pressure source (14) chargeable high-pressure accumulator (15), which emits a supply pressure, d) a pressure control valve assembly (26) which is indirectly actuated via the brake pedal, to the pressure supply and the pressure fluid supply reservoir

(12) is connected and outputs a bremspedalgesteu ^¬ Erten brake system pressure, e) solenoid valves for electronically controlled modification of the brake system pressure, f) electrically controllable pressure control valves (6a - 6d; 7a - 7d) for adjusting wheel-individual G) separating piston devices (5a - 5d), on a first separating piston surface (65a - 65d) with the by the pressure control valves (6a - 6d; 7a - 7d) are applied and forwarded to the pressure exerted by a second separating piston surface (75a - 75d) pressure to the wheel brakes (8 - 11), h) control elements (17a - 17d)) for mechanical control the separating piston devices (5a-5d), which can be brought into a force-transmitting connection with the second piston (3), and i) an electronic control unit

(16), characterized in that the second piston (3) limited retaining space (4) is designed as an annular space which surrounds the axis of symmetry defined by the axes of symmetry of the first and second pistons (2, 3) actuating axis radially.

2. Brake system according to claim 1, characterized in that the direction of actuation facing end face of the second piston (3) in a housing (20) formed depressurized space (19) is immersed.

3. Brake system according to claim 2, characterized in that the non-pressurized space (19) to the pressure medium reservoir (12) is connected.

4. Brake system according to claim 2 or 3, characterized in that the non-pressurized space (19) erungs sections of the Ansteuer- (17a, 17b) receives, which enable the mechanical power transmission from the second piston (3) to the separating piston devices (5a, 5b) ,

5. Brake system according to claim 4, characterized in that the drive elements (17a, 17b) are designed as round, sealed in the housing (20) and guided parallel to the actuating axis rods.

6. Brake system according to one of claims 1 to 5, characterized in that the first piston (2) in the second piston (3) defines a hydraulic follow-up space (34) which is formed as the first piston (2) radially encompassing annular space.

7. Brake system according to one of claims 2 to 6, characterized in that at the brake pedal (1) facing the end of the second piston (3) two seals (78, 79) are arranged side by side, which limit an annular chamber (80), wherein the first seal (78) seals the annular chamber (80) from the retention space (4) and the second seal (79) seals the annular chamber (80) from the atmosphere.

8. Brake system according to one or more of claims 1 to

7, characterized in that the retaining surface (4) limiting annular surface (82) is greater than the cross-sectional area of the first piston (2).

9. Brake system according to one or more of claims 1 to

8, characterized in that by actuation of the first piston (2) in the retention chamber (4) a hydraulic pressure can be built up.

10. Brake system according to one or more of claims 1 to

9, characterized in that the retaining space (4) via a hydraulic connection to the pressure medium reservoir (12) is connectable, which can be shut off by means of an electrically switchable valve (54).

11. Brake system according to one or more of claims 1 to

10, characterized in that the retaining space (4) via a valve (54) to the brake system pressure can be connected.

12. Brake system according to one or more of claims 1 to

11, characterized in that the first piston (2) in the second piston (3) limits a control space (23) in which it can build a controllable via a brake pedal hydraulic hydraulic control pressure.

13. Brake system according to one of claims 1 to 12, characterized in that the pressure regulating valve arrangement (26) is designed as a hydraulically controlled 3/3-way valve.

14. Brake system according to one of the preceding claims, characterized in that the electrically controllable pressure control valves (6a - 6d; 7a - 7d) for setting wheel-individual brake pressures each with a blocked in the de-energized state connection directly to the high-pressure accumulator (15) and with a In the de-energized state continuous connection are connected directly to the brake system pressure outlet of the pressure control valve assembly (26).

15. Brake system according to one or more of claims 5 to 14, characterized in that the Ansteuerungselemen- te (17a - 17d) in a force-transmitting connection with the second piston (3) can be brought and that between the second piston (3) and the Drive elements (17a - 17d) damping means (22a - 22d) are provided.